Generally, a waterproof sheet is used to waterproof of an acoustic unit for inputting and outputting voices and a breathing unit for balancing pressure. Example of acoustic units, which may be included a mobile phone include a speaker and a microphone. A waterproof test of the waterproof sheet may be performed by taking advantage of a characteristic of permeability indicating how much air passes through the waterproof sheet. That is, the waterproof test is performed by replacing water passing through the waterproof sheet with air.
There is a technique related to a method for measuring permeability based on a flow of air passing through a ventilation material (such as a waterproof sheet, for example) and a pressure difference between the back surface and the front surface of the ventilation material (see Japanese Laid-Open Patent Publication No. 2003-165172 (hereinafter referred to as Patent Document 1)).